Cell Biology: Microtubules and Plasma Membrane

Cell Structure and Function

• Cells vary in shape and size, with approximately 200 different cell types in the human body.
• Cells are the basic unit of life, organizing functions through organelles, which behave as “little organs.”
• Eukaryotic cells have membranous organelles; prokaryotic cells do not.

Organelles in Eukaryotic Cells

• The plasma membrane is a flexible barrier that separates internal and external environments, regulating material exchange and facilitating intercellular communication.
• The cytoplasm includes all content between the plasma membrane and nucleus, providing an environment for chemical reactions.

Cytoskeleton

• Composed of microfilaments, intermediate filaments, and microtubules, aiding cell shape, stability, and movement.
• Microfilaments (actin and myosin) support structure and facilitate movement.
• Intermediate filaments provide tensile strength and anchor organelles.
• Microtubules (made from tubulin) segregate chromosomes during cell division and form motile structures like cilia and flagella.

Centroles and Cell Movement

• The centrosome organizes microtubules and contains centrioles, crucial for mitotic spindle formation.
• Cilia (short) and flagella (long) enable movement in various cells, including respiratory tract cells and sperm.

Nucleus

• Contains DNA within a double membrane structure called the nuclear envelope, perforated by nuclear pores.
• Houses the nucleolus, responsible for ribosomal subunit production.

DNA Organization

• DNA exists as chromatin (loose form) for transcription and can be tightly wound into chromosomes for cellular division.
• The genome encompasses all DNA in an organism.

Ribosomes and Protein Synthesis

• Ribosomes are composed of rRNA and proteins, critical for translating mRNA to proteins.
• The central dogma of biology describes DNA transcription to mRNA, followed by translation to protein.
• Ribosomes can bind multiple transcripts, allowing for the rapid production of many proteins.

Endoplasmic Reticulum (ER)

• The rough ER (rER) has ribosomes for protein synthesis, continuous with the nuclear membrane.
• The smooth ER (sER) synthesizes lipids, stores calcium, and detoxifies drugs.

Golgi Complex

• Modifies, sorts, and packages proteins received from the rER into vesicles for transport.
• Cisternae are membranous folds facilitating these processes.

Lysosomes and Peroxisomes

• Lysosomes are membrane-bound sacs containing digestive enzymes for breaking down cellular materials.
• Peroxisomes oxidize molecules, metabolizing fatty acids and detoxifying harmful substances.

Mitochondria

• Bounded by two membranes, crucial for ATP production through aerobic cellular respiration.
• Mitochondria are maternally inherited and have their own DNA, replicating independently.

Cell Division

• Somatic cells undergo mitosis and cytokinesis, with interphase stages G1, S (DNA replication), and G2 (organelle replication).
• Mitosis proceeds through prophase, metaphase, anaphase, and telophase, ultimately leading to cytokinesis.

Telomeres

• Telomeres protect chromosome ends from degradation; they shorten with age.
• Telomerase maintains telomere length in cancer cells, contributing to rapid cell division.

Meiosis

• Meiosis produces gametes (haploid cells) through two rounds of division: Meiosis I and II.
• Crossing over occurs in Meiosis I, allowing genetic variation.

Membrane Structure and Fluidity

• The plasma membrane is a fluid mosaic model with a phospholipid bilayer, cholesterol, and embedded proteins.
• Membrane fluidity allows for protein and lipid movement, essential for many cellular functions.

Transport Mechanisms

• Simple diffusion allows nonpolar molecules to move across membranes passively without energy input.
• Facilitated diffusion utilizes ion channels and carrier proteins to transport charged or polar substances down their concentration gradients.
• Active transport requires energy to move substances against their concentration gradient.

Membrane Permeability

• Membranes selectively permit certain molecules; gases like O2 and CO2 pass easily, while polar molecules require specialized channels.

Summary of Cell Functionality

• Cells maintain homeostasis through various organelles, structures, and processes, ensuring efficient interaction with their environment and division for growth and repair.### Active Transport
• Moves solutes against concentration gradients across membranes.
• Sodium-potassium (Na+-K+) pump maintains high Na+ concentration outside the cell and high K+ concentration inside the cell.
• Na+-K+ pump operates through ATP hydrolysis, changing its shape to transport ions.

Types of Active Transport

• Primary active transporters directly use ATP to move solutes against their gradients.
• Secondary active transporters utilize electrochemical gradients established by primary active transporters to move solutes.
• Simultaneous transport by secondary active transporters categorizes into:
  • Symporters: both solutes move in the same direction.
  • Antiporters: solutes move in opposite directions.

Inhibition of Active Transport

• Cyanide inhibits ATP synthesis in mitochondria, thus stopping all active transport processes.

Vesicular Transport

• Vesicles transport products between organelles.
• Endocytosis: movement into cells via vesicles.
• Exocytosis: movement out of cells, termed secretion in some instances.
• Both processes require ATP hydrolysis.
• Transcytosis involves endocytosis followed by exocytosis to move substances through cells.

Types of Endocytosis

• Receptor-mediated endocytosis: imports specific molecules.
• Phagocytosis: “eating” of substances or invaders by immune cells, leading to lysosomal digestion.
• Pinocytosis: “drinking” of dissolved solutes, also known as bulk-phase endocytosis.

Osmosis

• The movement of water from low solute concentration to high solute concentration across a semi-permeable membrane.
• Aquaporins facilitate the efficient movement of water across membranes.

Tonicity and Cell Behavior

• Solutions may vary in solute concentration affecting cell behavior:
  • Hypertonic: higher solute concentration outside the cell, causing water to exit.
  • Hypotonic: lower solute concentration outside the cell, leading to water entering the cell.
  • Isotonic: equal solute concentration inside and outside the cell, resulting in no net water movement.

Clinical Application of Tonicity

• Intravenous (IV) fluids are typically isotonic to blood to maintain osmotic balance.
• Hypertonic solutions can lead to cell crenation, while hypotonic solutions can cause cell swelling.

Osmolarity

• Total concentration of solute particles in a solution, determining how a cell behaves in that solution.

Cell Division

• Somatic cells divide by mitosis, producing two identical daughter cells.
• Reproductive cells divide by meiosis, resulting in four nonidentical gametes.